You go into a players profile and look at your options coming down from the top left.
Click 'Overview' under 'Training' and you can select a new position for the player and also create a new training schedule for the player so that you can choose the area's he trains at most.
to ask them to adapt a new position then on the menu at the side under training it has overview click that then click new position then just click the new position you want them to adapt
To improve their stats set up some good training schedules and hire some good coaches. First team football also helps so if they are young loan them out or give them a few sub appearances now and then.